use ALL instead of ANY for fallback_for

[MeanderingProgrammer]

Feature Description

The definition of fallback_for is if any of these providers return 0 items, it will fallback to this provider.

I think rather than checking if any provider returns 0 items it might be more convenient to check that all providers return 0 items when setting up multiple completion providers.

As an example completion provider lets take crates.nvim which provides version completions in Cargo.toml files. When I use this provider ideally what happens is I do not get buffer items, only the ones returned from crates. Currently the way I would set this up is something like:

sources = {
  providers = {
    crates = { ... },
    buffer = {
      fallback_for = { 'lsp', 'crates' },
    },
  },
}

However this does not quite work since in Cargo.toml files the lsp provider returns 0 items so the buffer provider will run. Worse though is that in code files with an active LSP the crates provider returns 0 items so the buffer provider will run there as well now.

Ideally there would be a way to specify "only run this provider if non of the others have items". I think this is similar in concept to how nvim-cmp groups work. My idea to make this possible is to simply change the existing fallback_for to an all instead of an any. If this isn't possible than a new mechanism to make this possible would be great, maybe something like ignore_for, where meeting the fallback conditions makes the provider able to run but if any provider in ignore_for has items than it does not get run.